This invention deals generally with heat pipes and more specifically with the structure of a highly flexible heat pipe.
Traditional heat pipes are constructed with rigid metal casings and internal sintered wicks which, after manufacture, are expected to remain essentially in the same configuration as they were originally manufactured. Some such heat pipes have been constructed with thin casings to permit some reconfiguration, and there have been a number of patents for heat pipes which include flexible segments to enable repeated bending of certain parts of the heat pipe.
There are also a number of patents which have issued for heat pipes which are considered to be flexible in that their entire casings are constructed of thin flexible materials, and some of these patents include wicks which are also flexible. U.S. Pat. No. 5,560,423 by Larson et al discloses a flexible heat pipe with a thin metal sheet for one side of the casing and a thin plastic sheet for the other, with sheet screen wicking between them. U.S. Pat. No. 5,343,940 by Jean forms a flexible reheat pipe of laminated plastic material and keeps the surfaces so close together that the vapor space also acts as a capillary structure. Reinmuller (U.S. Pat. No. 4,842,045) suggests metal and elastomer composites among other materials for the envelope of a flexible condenser, but mentions no wick, and Fitzpatrick et al (U.S. Pat. No. 4,279,294) discloses flexible heat pipe bags with metal filled plastic and other materials used for the envelope and with a wick of fiberglass.
Nevertheless, none of these prior art patents address two significant problems with heat pipes. The first problem is ease of manufacture, without which a flexible heat pipe essentially remains a laboratory curiosity. The second problem is actually more significant, because it causes gradual deterioration of the vacuum within a heat pipe and therefore decreases the heat pipe""s useful life. Virtually every known H plastic is to some extent actually permeable to gas, particularly to hydrogen and helium. In most applications this has no significance whatsoever, but in heat pipes with thin plastic sheet casings and because of the very low internal pressure when a heat pipe is not operating, non-condensible gases do permeate into the heat pipe. It is the accumulation of non-condensible gases that eventually makes the heat pipe inoperable.
It would be very advantageous to have a truly flexible thin heat pipe which was not susceptible to permeation of gas into its casing and was flexible enough to actually wrap around small objects to cool them. An even more advantageous configuration for a heat pipe would be a continuous length of highly flexible, flat heat pipe with an adhesive preapplied to one outside surface and seals between sections at a regular intervals. Such a structure would, for all intents and purposes, be a heat pipe in the form of a length of tape.
The present invention is essentially a very thin and very flexible heat pipe which, when coated with adhesive on one outside surface, can be used as if it were tape. That means, for instance, that if an integrated circuit requires cooling, the heat pipe can be adhered to the integrated circuit and to a remote heat sink, and the heat from the integrated circuit will be efficiently transferred to the heat sink even if the heat sink is on a panel which is moveable relative to the integrated circuit.
The preferred embodiment of the heat pipe of the present invention is only about 0.120 inch thick, and it comprises five major layers. The central layer is a coarse screen which acts as a separator to establish the heat pipe vapor space by separating two layers of copper felt wick, one on each side of it the screen layer. The other two layers, which are sealed together around their edges, form the envelope of the heat pipe around the wick and the separator, and the envelope walls are themselves composed of multiple layers of metal, adhesive, and plastic.
The two envelope walls of the preferred embodiment start with an inside layer of polypropylene which acts as a heat activated bonding agent. That is, when the edges of two envelope walls are pressed together and heat is applied, the two envelope walls seal together because their inner layers of if polypropylene bond together. The next layer of the envelope walls is a very thin layer of polyethylene terepthalate. This material acts as an adhesive to bond the next layer of copper foil to the previous polypropylene layer. Then there is another layer of polyethylene terepthalate adhesive and another layer of copper foil on the outside of the envelope. Other layers can also be added for particular applications. For instance a tedlar layer can be used to furnish better external abrasion resistance, or an adhesive layer can be added to aid in attachment and installation of the heat pipe.
The two copper foil layers are used to improve the reliability and life expectancy of the heat pipe, and yield better results than a single layer with a thickness equal to the total of the two layers. Based on the understanding that all foil layers have occasional and random pinholes in the original sheets, the use of two layers reduces the likelihood of vacuum leaks because of the very low probability that two such pinholes in separate sheets of foil will actually align in A the final structure. Additionally, bonding of plastic layers to both sides and between the metal foil layers reduces the likelihood of stress concentrations and resultant pinhole formation through the metal foil layers.
The flexible heat pipe of the invention thereby has a reliably leak tight envelope even though the thickness of each wall of the envelope is less than 0.010 inch. Those thin walls along with two copper felt wicks of only 0.10 to 0.040 inch thickness and the coarse polypropylene separator screen about 0.040 inch thick permit the structure to be extremely flexible and yet, when loaded with a suitable fluid, function as a very efficient heat pipe.